Q: How do I get rid of the "Dum-m-m-m-m-m-m" start up sound?

That sound means it is all well.  3 short beeps, repating in a pattern mean memory has issues, and other sounds means other things.

I use this to make the sound softer, but audible (my college-bound nephew uses it because te might start the units while in the college library).  Note that is only works through Snow Leopard (10.6.x).  Other solutions needed for Lion and beyond.

http://www.macupdate.com/app/mac/16425/startupsound.prefpane 

Use Psst. It's supposed to be able to control the boot chime sound volume even in Lion.

Note, it I do not recommend turning the chime off completely because the boot chime is used by POST as an error indictor reporting mechanism.

Power On Self-Test Beep Definition - Part 2

Psst ... I will change my advice in the future.